The present invention relates to a circuit arrangement for the automatic de-excitation of a hysteresis motor whose reactive power intake is compensated by capacitors and whose stator winding, which is fed from a three phase mains via a frequency converter, magnetizes, with its rotating field, the rotor made of ferromagnetic material so as to produce a permanently magnetic excitation which remains in existence in the rotor in the form of residual magnetic poles even after the motor is disconnected from the three phase mains.
The rotor of a hysteresis motor is made of ferromagnetic material with defined hysteretic properties. During standard synchronous operation the hysteresis motor behaves similarly to a synchronous machine in greatly underexcited operation. However, whereas in the synchronous machine the excitation field is generated by an excitation coil to which a direct voltage is applied, in the hysteresis motor the rotating field of the stator produces permanently magnetic poles in the, for example, disc-shaped rotor, with the field intensity of these poles being less than that of the excitation field produced by the direct current windings in the synchronous machine.
The excitation of the rotor of a hysteresis motor cannot be switched off without difficulty, i.e., it remains in existence in the rotor in the form of residual magnetic poles even after the motor has been disconnected from the three phase mains.
Another property of the hysteresis motor is its unfavorable power factor (cos .phi..apprxeq.0.3) which in systems comprising a plurality of such motors leads to a considerable reactive power load on the three phase current supply mains. For that reason, compensating capacitors are provided to effect an exchange of reactive power within the system including the motor and the capacitor, and the three phase mains covers only the requirement for actual operating power. If the capacitance of the compensating capacitor at operating frequency is matched so that the reactances of the motor and of the capacitor have the same value, the mains carries the load of a matched parallel resonant circuit which is attenuated only by the motor losses and by the actual power transferred to the shaft. If the system including the motor and the capacitor is disconnected from the mains, the motor, whose EMF remains in effect, is driven by its inertial mass and operates as a generator and the parallel resonant circuit fed from the mains becomes a weakly attenuated series resonant circuit at the EMF of the motor. The generated current, which is limited by the active resistance, causes the rotor to be magnetized more strongly and increases the EMF which again drives a higher current through the series resonant circuit until, within a few milliseconds, motor current and terminal voltage reach a multiple of their operating values. This process of self-excitation must be avoided under all circumstances since it is not only a danger to the energy supply devices in the form of excess voltage and current loads, but can also lead to destruction of the motor due to the increased load on the bearings.
It is known to employ a voltage monitor to monitor the motor voltage at a central location of the energy supply system and to short-circuit the motor terminals and thus also the terminals of the compensating capacitors when such a malfunction occurs. This does eliminate the series resonance conditions but the motor rotor is only partially de-excited so that upon elimination of the short-circuit the self-excitation process may start up again due to residual magnetization of the rotor. It is therefore necessary to de-excite hysteresis motors as completely as possible. Even if operating conditions require disconnection of the motor, it should be attempted to obtain as complete a de-excitation as possible since the re-connection of excited hysteresis motors to static frequency converters is difficult.